Smokeless powders have been developed since the 19th century to replace traditional black powder, which generates substantial amounts of smoke when fired. The most widely used smokeless powders are nitrocellulose-based. Nitrocellulose is obtained by using nitric acid to convert cellulose into cellulose nitrate and water according to a general reaction:3HNO3+C6H10O5→C6H17(NO2)3O5+3H2ONitrocellulose-based smokeless powder is then obtained by treating the thus obtained nitrocellulose by extrusion or spherical granulation, with or without solvent, two techniques which are well known to the persons skilled in the art.
Various improvements have been developed since the first discovery of nitrocellulose, by addition of further components, such as nitroglycerine and/or nitroguanidine allowing an increase of the detonation velocity. Pure nitrocellulose propellant is referred to as single-base propellant, and double- and triple-base propellants refer to compositions comprising nitrocellulose and one or two additional energetic bases, respectively, typically blasting oils such as nitroglycerine, nitroguanidine, or secondary explosives.
Nitrocellulose, as most nitrate esters, is prone to self-ignition as a result of thermal degradation due to the weakness of its O—N bond. When employed as an ingredient of propellants or other explosive compositions, the spontaneous ignition of nitrocellulose has caused serious accidents. It is obviously vital to inhibit or slow down this degradation for safety reasons but it is also important to retain the initial properties of the energetic composition. Degradation usually leads to gas emissions, heat generation and reduction of molecular mass affecting negatively the material structure and ballistic properties.
The decomposition of nitrocellulose usually starts with a bond scission or hydrolysis, generating alkoxy radicals and nitrogen oxide (NOx) species (cf. FIG. 1). The radicals further react, generating more radicals, speeding up the degradation process, and ultimately lead to chain scission accompanied by heat generation. In order to prolong the service life of the propellants, stabilisers are added to the energetic mixture in order to scavenge these radical species and slow down the degradation pattern.
All conventional stabilisers used to date for nitrocellulose-based propellants belong to (a) aromatic amines (e.g., diphenylamine, 4-nitro-N-methylamine) or (b) aromatic urea derivatives (e.g., akardite, centralite) and are or produce toxic and/or potentially carcinogenic species at some point during the propellant's lifetime. For example, the most widely used stabilisers to date are diphenylamine, akardite, and centralite. These compounds, however, form carcinogenic derivatives such as N-nitrosodiphenylamine (cf. FIG. 2(a)) or N-nitrosoethylphenylamine.
Hindered amines, such as triphenylamine, reduce the formation of N—NO groups, but fail to stabilise nitrocellulose satisfactorily. Conventional hindered phenols used in the plastics industry have been tested and at short term stabilise nitrocellulose with little to no N—NO formation. The phenols are able to trap the alkoxy radicals generated during the degradation of nitrocellulose and thus form new, relatively stable alkoxy radicals, by delocalisation of an electron at the foot of electron-rich, hindered groups as illustrated in FIG. 2(b). Long term stability is, however, not always guaranteed, probably due to rapid phenol depletion and relative stability of the newly formed alkoxy radicals.
There thus remains in the field of solid propellants a need for stabilisers allowing long term stabilisation of nitrocellulose based propellants, fulfilling at least STANAG 4582 (Ed.1) and which do not produce carcinogenic and/or mutagenic by-products. The present invention proposes a family of stabilisers fulfilling both requirements above. These and other advantages of the present invention are presented in continuation.